fix: import legacy projects without wrapper folder

hub/test/integration/legacy-project-import.test.ts

@@ -29,7 +29,7 @@ describe("legacy teaching-material project import", () => {
 
   afterAll(async () => prisma.$disconnect());
 
-  it("imports each legacy project as an unbound resumable project under its old folder path", async () => {
+  it("imports each legacy project as an unbound resumable project under its original folder path", async () => {
     const sourceRoot = await temporaryRoot("cph-legacy-source-");
     const workspaceRoot = await temporaryRoot("cph-legacy-target-");
     const projectSource = join(sourceRoot, "物理", "M-243-牛顿力学");
@@ -70,7 +70,7 @@ describe("legacy teaching-material project import", () => {
     });
     expect(project.name).toBe("牛顿力学");
     expect(project.folder?.name).toBe("物理");
-    expect(project.folder?.parent?.name).toBe("旧教学资产");
+    expect(project.folder?.parent).toBeNull();
     expect(project.groupBindings).toEqual([]);
     await expect(readFile(join(imported.workspaceDir, "chapters", "lesson.typ"), "utf8"))
       .resolves.toBe("= 牛顿第二定律\n");
@@ -116,6 +116,36 @@ describe("legacy teaching-material project import", () => {
     await expect(prisma.project.count({ where: { id: imported.projectId } })).resolves.toBe(1);
   });
 
+  it("places a legacy project without a source folder in the system Inbox", async () => {
+    const sourceRoot = await temporaryRoot("cph-legacy-source-");
+    const workspaceRoot = await temporaryRoot("cph-legacy-target-");
+    const projectSource = join(sourceRoot, "top-level-project");
+    await mkdir(join(projectSource, "workspace"), { recursive: true });
+    await writeFile(join(projectSource, "workspace", "project.toml"), "title = \"顶层项目\"\n");
+    await writeFile(join(projectSource, "project.json"), "{\"id\":\"top-level\"}\n");
+
+    const state = await importLegacyProjects({
+      prisma,
+      organizationId: DEFAULT_ORG_ID,
+      actorFeishuOpenId: "ou_legacy_owner",
+      workspaceRoot,
+      sourceRoot,
+      stateFile: join(workspaceRoot, "state.json"),
+      projects: [{
+        legacyId: "top-level",
+        name: "顶层项目",
+        folderPath: [],
+        sourceRelativePath: "top-level-project",
+      }],
+    });
+
+    const project = await prisma.project.findUniqueOrThrow({
+      where: { id: state.projects["top-level"]?.projectId },
+      include: { folder: true },
+    });
+    expect(project.folder).toMatchObject({ name: "Inbox", kind: "SYSTEM_INBOX" });
+  });
+
   it("rejects symlinks instead of importing paths outside the staged project", async () => {
     const sourceRoot = await temporaryRoot("cph-legacy-symlink-");
     const workspaceRoot = await temporaryRoot("cph-legacy-target-");